Operation of a photonic network may include changing the signal wavelength assigned to a wavelength path, switching the modulation/demodulation mode, a nonlinear compensation mode, or an error correction (forward error correction; FEC) mode, and reallocating network resources. Since the photonic network is in operation, such switching and reallocation are desirably performed with minimal downtime. Therefore, the timing of switching respective processes must be synchronized at the transmitting end, relay nodes, and the receiving end.
According to a conventional scheme, a control signal channel is assigned to a wavelength that is beyond the wavelength band assigned to a main signal channel and, by wavelength multiplexing and transmitting the main signal channel and the control signal channel from the transmitting end, the relay node and the receiving end are notified of the control timing. According to another scheme, for example, as in the case of Synchronous Digital Hierarchy (SDH) and an Optical Transport Network (OTN), if overhead exists that is included in a digital frame structure for signal transfer of the main signal, this is used for giving notification of the control timing.
According to some schemes, to manage and control a network, for example, the intensity, the frequency, or the polarization state of an optical signal is modulated to superimpose a control signal on a main signal for transmission (for example, refer to U.S. Pat. No. 7,580,632; US Patent Publication Nos. 2012/0014695 and 2012/0141130; Heismann, Fred, et al, “Signal Tracking and Performance Monitoring In Multi-Wavelength Optical Networks”, 22nd European Conference on Optical Communication—ECOC '96, 1996, pp. 3.47-3.50; Tanimura, Takahito, et al, “Superimposition and Detection of Frequency Modulated Tone for Light Path Tracing Employing Digital Signal Processing and Optical Filter”, Optical Fiber Communication Conference and Exposition (OFC) and the National Fiber Optic Engineers Conference (NFOEC) 2012, (USA), Mar. 4, 2012; and Takeshita, Hitoshi, et al, “Study for signal format independent and degradation-free optical path identifier based on SOP modulation for multi-degree CDC-ROADM systems”, 2012 Proceedings of the Institute of Electronics, Information and Communication Engineers (IEICE) CONFERENCE 2, (Japan), Mar. 20, 2012, p. 436).
However, in a convention scheme of giving notification of the control timing through wavelength multiplexing of a main signal channel and a control signal channel, separate transmitters and separate receivers are prepared for a main signal and a control signal and therefore, a difference may occur in length of the optical transmission path. Since wavelengths are different between the main signal channel and the control signal channel, a skew occurs between the main signal and the control signal consequent to wavelength dispersion of optical fibers. If the wavelength of the control signal channel is beyond the band of an optical amplifier such as an erbium doped fiber amplifier (EDFA), a relay node performs regenerative relaying accompanied by optoelectronic conversion and electrooptic conversion for an optical signal of the control signal channel and therefore, a relaying process of the control signal takes time. Consequent to these factors, the timing deviates or temporally varies when the main signal and the control signal transmitted from the transmitting end reach the relay node and the receiving end, causing a problem of increased error of the control timing between the transmitting end and the relay node/the receiving end.
On the other hand, in a conventional scheme of using an overhead byte of the main signal for giving notification of the control timing, the readout of the overhead byte is enabled after properly setting a modulation mode and a bit rate, and communicating the main signal. Therefore, problems arise in that a receiver for the main signal must be prepared in the relay node and a waiting time for the communication of the main signal is required.